Power Metal Oxide Silicon Field Effect Transistors (MOSFETs) that require higher voltages rely on super junction structures that are generated by deep trench etching and an epitaxial (EPI) filling process. FIGS. 1A-D illustrate in a general way the process used to create these super junction structures. In FIG. 1A, N− epitaxial layer 104 is grown on an N+ substrate 102. In FIG. 1B, a thick layer of silicon dioxide 106 is grown or deposited on the surface of N− epitaxial layer 104 to act as a hardmask during etching. Silicon dioxide layer 106 is patterned for the creation of trenches, followed by an etching step to form trenches 107. In one example, trench 107 is 4 microns wide and 50 microns deep, extending almost to substrate 102. It will be understood that these figures are not drawn to scale, but are provided as illustrations only.
In FIG. 1C, P− epitaxial layer 108 is grown on the inner surface of trenches 107 to form deep silicon buried pillars in N− epitaxial layer 104. As shown, the P− epitaxial layer 108 is grown to overfill trenches 107. As seen in FIG. 1D, overfilled portions of epitaxial layer 108 are removed in a chemical-mechanical polishing (CMP) process that uses the hardmask formed by silicon dioxide 106 as a stopping point. This figure illustrates the super junction structures after the removal of silicon dioxide layer 106. In conventional power MOSFETS, there is a trade-off relationship between the on-state resistance (RON) and the breakdown voltage (BV). The super junction structures allow a higher voltage MOSFET with a lower resistance than otherwise possible and are important in high voltage power devices. The high aspect ratio, substantially perpendicular, trench profile is preferred in terms of trade-off between on-resistance and breakdown voltage; however, these high aspect ratio, substantially perpendicular deep trenches provide a difficult environment with regard to epitaxial growth, as will be explained in greater detail below. Improvements in epitaxial growth in deep trenches are needed.